docs: def validate_units(self):
          """Ensure that wavelenth unit belongs to the correct class.
          There is no check for throughput because it is unitless.
  
          Raises
          ------
          TypeError
              Wavelength unit is not `~pysynphot.units.WaveUnits`.
  
          """
          if (not isinstance(self.waveunits, units.WaveUnits)):
              raise TypeError("%s is not a valid WaveUnit" % self.waveunits)